nx_av1_intra.nx

nx_av1_intra.nx -- AV1/AV2 non-directional intra prediction. Part of the residual path: before any coefficient is added, a block is PREDICTED from its already-decoded above row and left column. These are the modes that need no angle -- DC, vertical, horizontal, Paeth and the three smooth variants -- and together they cover most real intra blocks. AVAILABILITY CHANGES THE FORMULA, NOT JUST THE INPUT. DC has FOUR distinct cases: both edges present, only above, only left, and neither. With neither it is not zero and not an average of nothing -- it is the mid-grey 1 << (bitdepth - 1). A decoder that treats a missing edge as zeros predicts black along the top and left of every frame, which looks like a vignette rather than a bug. THE SMOOTH WEIGHTS SUM TO 256, NOT 255. Each smooth predictor blends a weight w against (256 - w) and rounds with +128 >> 8 (or +256 >> 9 for the two-axis form). Using 255 as the complement makes constant input predict slightly-wrong constant output -- a fraction of a level per pixel, which accumulates across a frame of intra blocks and shows as a gradient. PAETH PICKS, IT DOES NOT AVERAGE. It selects whichever of left, above or above-left is nearest the linear estimate. The tie order matters: left beats above, and above beats above-left. EVERY LOCAL IS DECLARED ONCE. nx_cc uses FLAT FUNCTION SCOPE -- re-declaring a name inside a sibling branch desyncs its parser (proven here 2026-07-31: a `let below` in two exclusive branches produced 'unexpected operator token at expression start'). Params also travel in an array rather than as ten positional arguments. genealogy_id: av1_spec_7_11_2_intra_prediction lineage_id: nx_av1_intra_v1 license_tier: ORIGINAL